TL;DR - cheap guitar goes out of tune from one string bend. looking for some tips to get it better before going (if at all) to a set-up.

hi fellas
i bought this really cheap strat knockoff for 60$. the guy i bought it from said he barely touched it, and that the strings are the same strings since he bought it in 2006. it plays great and sounds alright (which is more than enough), BUT,

my only problem with with the guitar is the fact that it does not stay in tune at all. AT ALL.. one bend will make a string go around a half step down. no joke. so its pretty much impossible to play  
here's the thing:

i've never took my guitar to a setup (my main guitar that is, which i own for around 3 years now) and so i dont know what to expect. i never needed to because it worked perfectly (PRS ce ftw) but obv this new guitar doesn't. my questions:

1. what can i tinker with to get her to stay in tune? and how? (if you have guides links would be helpful too )

2.is there a point taking it to a set up?  A.I. what exactly is the guy going to do? will it actually fix it? 

Well start with changing the strings, decking the trem if it has one and lubing the nut with graphite (pencil lead).
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Seeing on how you already have a good working guitar to play, this "cheapy" might be good to learn some guitar set- up techniques. Def' worth knowing how to set-up your own guitars. 
There are tons of setup guides, there's even one stickied at the top of this forum, and a trillion youtube videos to check out. Lots of help out there. 
If you've never set up a guitar before, it might be a little frustrating to try to fix one that's in really bad shape. 

Here's what I would do:
-Change the strings
-Check that the nut is slotted for that gauge of string and string trees are lined up straight and not catching, check tuners for slop
-Lubricate the nut 
-Check trem setup, claw/back screws for tension and smooth reset, front studs/screws for height, wear, and tightness, trem arm for play, make sure block isn't catching ball ends
-Check relief and neck straightness, adjust as needed 
-Set string height intonation, check saddles for burrs, set witness points 
-Check for fret wear, polish/crown as needed

Then the extra stuff that won't effect tuning stability but is part of a good setup:
-Check pots, switches, jack, etc. for shorts and function, clean/tighten/replace as needed
-Check grounding wire and shielding
-Adjust pickup heights and/or pole pieces as requested 
-Clean fretboard

That's sort of a "full service" setup, give or take a few items. It's hard to say that a setup will definitely fix your problem, but unless there's something structurally wrong with your guitar, a competent tech should be able to get it working much better. 
Most issues with guitars that go out of tune have to do with the nut, the strings and occasionally the bridge.
The nut needs to be cut properly to allow the strings to slide (and not catch) during bends or trem manipulation.
If your strings are old and rusty, you probably need to change them. If they're brand new, you need to make sure that they've been properly stretched.
And your bridge needs to be properly adjusted.

Too-high action will make it sound as if your strings are out of tune, as will pressing down too hard on the strings (especially if you've got tall frets). Frets that are flattened and not properly crowned will have contact points that are "off" and will make your chords sound strange.

If NONE of those is your issue, then it might be time to look at your tuners. But even on cheap guitars most tuners are just fine.